There are a number of prior art systems utilizing ultrasonic energy for determining the contents of a container. For example, Willis et al. U.S. Pat. No. 3,834,233 utilized a stilling pipe mounted within a stand pipe to direct sound energy down into the tank and receive an echo from the surface of the contents. To compensate for inaccuracies due to changes in the velocity of sound in air, a second receiver is position a fixed distance from the transmitter and both receive signals are processed to cancel the effects of any velocity variation in the speed of sound. A generally similar arrangement is shown in Massa U.S. Pat. No. 4,210,969 which utilizes a microprocessor located at the top of the tank and projecting a beam onto the fluid surface for determining the actual level of the fluid. A fixed target located a known distance from the ultrasonic transducer is utilized to compensate for temperature or gradation variations caused by presence of chemical vapors or gradations or temperature changes in the gas/air above the fluid level and a microprocessor system is utilized for making the measurement and for calibrating.
According to the present invention, two independent measurements of the same paths are made, and, since the geometry and the base leg of the triangle is known, the speed C of ultrasonic waves drops out. A pair of transducers are mounted, above or preferably below the liquid level and a fixed predetermined distance apart. One or both of the two transducers may be activated to transmit ultrasonic pulses to be reflected along the paths described above and measurements of the times between the two paths is utilized to calculate the height H of the liquid using the formula: ##EQU2## where S equals the known distance between transducers, T is the time for ultrasonic energy to vertically travel to the liquid surface and T' is the time for ultrasonic energy to travel between transducers via a reflection path at the liquid surface.
The invention provides a simple, inexpensive and highly accurate method of measuring fluid levels in tanks and is especially useful in measuring fluid levels in tank farms under microprocessor control.
According to the present invention, a pair of transducers are positioned at a fixed distance apart and preferably below the surface of a liquid whose level is to be measured. Both transducers are microprocessor controlled and provide two independent measurements of range and since the geometry is known, it drops out of the measurement because the speed of sound over both paths is the same. At the interface between the liquid level and air or whatever gas is above the liquid, there is substantially complete acoustic reflection since the ratio of specific acoustic impedances between the two media determines the degree or amount of acoustic energy that leaves the liquid or enters the liquid if the transducers are located above the liquid surface. The invention thus avoids any effect of a temperature gradient. In a room, a temperature gradient of one degree per farenheit per foot means that the velocity can vary a large amount with each degree of change. Where extreme precision of measurement is required, the temperature gradient can drastically effect the accuracy. For example, the bottom of a tank in a fuel tank farm is at ground temperature or at about fifty-five degrees. The top of the fuel is somewhere near the average air temperature for the day, and the top will be hot or near the liquid temperature so that the temperature gradient in the chemical is made up of gases and will also vary with the quantity in the tank.